1. Field of the Invention
The present invention relates to a method for forming cored passageways within cast metal articles.
2. Description of the Prior Art
Cores are commonly used in connection with metal casting operations for forming spaces, such as fluid passages, within cast metal articles.
Sand cores are most often employed because of their low cost. However, the application of sand cores is limited to applications in which the length to thickness ratio of the core is fairly small. That is, sand cores having a very small cross-section and a very long length are extremely fragile and easily broken during handling and/or casting, making the use of sand cores in such applications commercially impractical.
An example of where a sand core would be unsuitable is in the formation of oil galleys within the engine block and cylinder heads of an internal combustion engine. These oil galleys are formed as very long and relatively narrow passages that extend virtually the entire length of the casting. A sand core of this configuration is much too fragile and is not commercially employed in this or similar applications.
A proposed alternative to the sand core is disclosed in the U.S. Pat. No. 2,991,520 to Dalton, granted Jul. 11, 1961. This patent suggests using a sleeve of refractory material disposed about a tube or rod as the core. Following casting, the rod is either manually removed or dissolved by an acid solution and the sleeve pulled out of the casting to leave the desired passageway. Although this method of coring should in theory work, attempts at practicing the invention as disclosed have shown that the sleeve is very difficult to remove from the resultant cast article following casting, thus rendering use of such a coring technique commercially impractical as well. In particular, it has been found that when the molten metal is cast into the cavity and around the sleeve-covered core, the molten metal has a tendency to enter the interstices of the woven sleeve. Upon solidification of the molten metal, the sleeve is thus caused to remain stuck or adhered to the cast article, making removal unacceptably difficult. Dalton, teaches that the sleeve will simply collapse inwardly following removal of the core or support element from within the sleeve, implying that the sleeve does not remain attached to the casting as was found in practice. Dalton relies on this inward collapsing for the easy removal of the sleeve. Dalton does not address the difficulties of removing a sleeve that remains attached to the passageway walls of the casting following removal of the inner core support.
Because the present coring technology precludes using cores to form long and relatively narrow passages in cast articles, the industry has turned to other and more costly alternatives.
For instance, it is common practice to rifle drill the oil galleys into cylinder blocks and heads in a separate machining operation following casting. This process is more expensive than coring and presents problems particularly with aluminum engine blocks and cylinder heads since the drilling tool tends to drift or wander as it is extended into the casting. Thus, several drilling operations are usually required to make a single oil passage and further adds to the expense of forming such passages.
Thus, there are these and numerous other applications in which coring would be the preferred way of forming a passageway within a metal casting. However, because of the present coring technology available to the industry, coring is commercially impractical and other more costly alternatives must instead be employed. Accordingly, there is a need in the industry for a coring process which may be employed in applications where a long and relatively thin core is required.